This invention relates to a process for preparing microporous fibers from poly(etheretherketone)-type polymers. In a preferred embodiment, this invention relates to a process for preparing microporous poly(etheretherketone)-type hollow fiber membranes. Such hollow fiber membranes are useful in the treatment of liquids by the membrane separation processes of ultrafiltration, microfiltration, pervaporation, membrane distillation, and reverse osmosis. The hollow fiber membranes of this invention are also useful as microporous supports for hollow fiber composite liquid or gas separation membranes.
Poly(etheretherketone)-type polymers are high performance thermoplastics which possess high glass transition temperatures, high crystalline melting points, high thermal stability, and high solvent resistance. These properties make poly(etheretherketone)-type polymers useful for a number of applications, including the fabrication of high strength fiber reinforced composite materials. Poly(etheretherketone)-type polymers are also desirable polymers for membranes used in liquid separations, particularly membrane separation processes which involve treatment of strong organic, acidic, or basic solvents at elevated temperatures.
The very properties which make poly(etheretherketone)-type polymers desirable materials for use in applications which require high strength or solvent resistance also make the polymers very difficult to process. One typical method of preparing fibers involves dissolving the polymer material in a solvent or a mixture of solvent and non-solvent, extruding the blend into hollow fibers, and immersing the extruded fibers in a coagulation bath. However, poly(etheretherketone)-type polymers are extremely solvent resistant and are poorly soluble in all common solvents Therefore, for example, to form membranes, poly(etheretherketone) is typically dissolved in very strong organic acids such as concentrated sulfuric acid to sulfonate the poly(etheretherketone), which makes the polymer soluble in common solvents such as dimethylformamide and dimethylacetamide. The problem with this process is that the solvent resistance of sulfonated poly(etheretherketone) is less than that of the unsulfonated polymer. Furthermore, sulfonated poly(etheretherketone) swells in aqueous solutions, which adversely affects membrane performance in aqueous separation applications.
What is needed is a process of preparing microporous poly(etheretherketone)-type fibers using solvents or mixtures of solvents and non-solvents which do not chemically modify or degrade the polymer during extrusion so that the high strength and solvent resistance of the polymer is retained by the fabricated fibers.